A systematic investigation of TMB substrate composition for signal enhancement in ELISA.

3,3',5,5'-Tetramethylbenzidine (TMB) remains one of the most widely utilized chromogenic substrates for horseradish peroxidase (HRP) in colorimetric immunoassays, including enzyme-linked immunosorbent assays (ELISA). Despite its introduction into ELISA workflows over four decades ago, limited research has been conducted to systematically optimize TMB substrate formulations. Recent advancements in the field have proposed innovative approaches to enhance HRP catalysis, such as the use of deep eutectic solvents and ionic liquids, alongside investigations into the chemical properties of TMB and its analogs to identify more efficient alternatives. However, the development of stable and high-performance TMB solutions for clinical diagnostics requires a comprehensive understanding of how formulation parameters influence signal intensity and stability. In this study, we address these gaps by conducting a systematic evaluation of key factors affecting TMB substrate performance, including buffer pH, composition and molarity, specific ion effects, incorporation of organic solvents, and the use of polymer stabilizers. Additionally, novel strategies for signal amplification, identified through an extensive review of literature and patents, were experimentally tested. Based on these findings, we developed an optimized TMB formulation comprising 0.2 mol/L sodium citrate buffer (pH 4.5), 5 % DMSO, 0.37 mmol/L CaCl, 0.4 mmol/L 2-hydroxy-β-cyclodextrin, 0.8 mmol/L TMB, and 1.3 mmol/L HO as final concentrations. The performance of this optimized formulation was evaluated in comparison to previously reported formulations from the literature.